The present invention generally relates to shielded electronic devices and printed circuit boards. More specifically, the present invention provides an EMI shield coupled to a surface of the printed circuit board and an EMI shield that is formed within the printed circuit board.
Electronic products emit electromagnetic radiation, generally in the range of 50 MHz to 3 GHz, but not limited to this range, especially in light of the many advances in high-speed microprocessor design and the rapidly increasing capabilities of high-speed networking and switching. The problem of emittance of electromagnetic radiation is not new to designers of electronic equipment. Indeed, significant efforts are taken to reduce electromagnetic interference (EMI) and virtually every county has a regulating agency (FCC in the U.S., for instance) that regulates the marketing and sale of electronic equipment that do not pass stringent requirements for EMI, whether radiation is emitted or intercepted (also called susceptibility) by the electronic equipment
Conventional EMI shielding solutions include the use of conductively painted plastic outer housings, conductive gaskets, and metal cans that are affixed to the printed circuit board by soldering or similar methods. In virtually all cases, the existing solutions are expensive and add to the cost of manufacturing electronic equipment such as cell phones, personal digital assistants, laptop computers, set-top boxes, cable modems, networking equipment including switches, bridges, and cross-connects.
More recently, technology for the metalization of polymer substrates has been attempted. For example, Koskenmaki (U.S. Pat. No. 5,028,490) provides a polymer substrate that is layered with aluminum fibers and sintered to form a flat material with a metal coating that is intended to provide effective EMI control (also a legal requirement of the FCC and other foreign entities and generally referred to as electromagnetic compliance or EMC). Unfortunately, the material has proven to be expensive, difficult to use, and subject to inferior performance due to cracking of the aluminum fiber layer. The Koskenmaki aluminum fiber metal layers had limitations on the ability to withstand a thermoforming process due to the typical tight radius used in the thermoforming molds.
U.S. Pat. No. 5,811,050 to Gabower, the complete disclosure of which is incorporated herein by reference, has provided an alternative approach wherein the thermoformable substrate (any number of different kinds of polymers) is first formed then metalized. This approach offers the advantage of not subjecting the metallized layer to the stresses created during molding. The product has been shown to be highly effective and relatively low-cost.
The major methods of providing for a conductive coating or layer on a substrate include (1) selective electroless copper/nickel plating, (2) electroless plating, (3) conductive paints and inks, and (4) vacuum metalization. Collectively, these are referred to herein as “metalization methods.” In each of these applications, either a planar or formed substrate of metal or plastic is “treated” to form a conductive shield. The ultimate quality, performance, and cost for each method varies widely but ultimately a metalized thermoformable shield is formed into an (1) integral solution that surrounds the printed circuit board in some manner (e.g., “enclosure” level solution), (2) formed into a compartmentalized shield that fits on the surface ground traces of the PCB (e.g., “board” level solution), or (3) formed into smaller shields that fit over individual components again using the surface ground traces (e.g., “component” level solution).
When it comes to EMI shielding at the printed circuit board at the component level, the conventional solution is to place a conductive surface of an EMI shield in contact with the surface ground traces either (1) directly by metalizing a shield surface and placing it in contact with the ground trace or (2) by metalizing the “outside” surface (from the perspective of the component being shielded) and then using some method of attachment that connects the ground trace with the metalized outside surface. The purpose of the surface ground traces, based upon the historical use of soldered metal cans, is to provide a point of contact between the metal can and printed circuit board that can be subject to standardized surface mount technology (SMT) solder reflow processes that ultimately provide a solid and permanent connection between the metal can shield and the printed circuit board. While the metal can and surface ground trace become grounded in at least one point to the ground plane, the amount of peripheral contact between the shield and metal can is largely for the purpose of achieving a tight mechanical seam.
The resultant assembly of the shield onto the PCB provides adequate shielding for electronic components such as integrated circuits, oscillating clock chips and similar devices in numerous applications. However, as the frequency of chips increase (e.g., greater than 1 GHz) and the data transmission rates increase, the creation of errant EMI radiation becomes much easier and more harmful to circuits and components located nearby. Indeed, with the increasing density of chips, the subject of immunity (of one chip relative to another) becomes all the more important. Thus, in general, conventional solutions will increasingly find themselves inadequate for purposes of immunity and indeed, radiated emissions, may also become an increasing issue. Moreover, for microwave devices, especially those that operate of have harmonic frequencies above about 10 GHz., radiated emissions will be a significant concern.
Improving the EMI performance of the metallized thermoform requires an examination of the structure of the shield/board interface. FIG. 1 illustrates a conventional shielding solution in which a PCB 10 with an emitting electronic component 12, such as a semiconductor chip, and EMI shield 14 are depicted (not to scale). The EMI shield 14 is placed on surface ground traces 16 on the surface of the PCB through soldering that provide for electrical continuity. Radiation 18 from the chip can emerge through the PCB substrate (glass/polymer structure, as for instance, flame retardant 4 layer board—e.g., FR4). In FIG. 1, radiation 18 is shown as bouncing off a ground plane 20 and emerging either into the environment or adjacent to another chip (not shown). It should be appreciated that the radiation fields are comprised of very complex combinations of both electric and magnetic fields that are bouncing off chip and shield structures forming very complex fields with many resonances. These resonances can be very strong in terms of field strength and can easily be observed at frequencies that are troublesome from an EMC perspective.
In general, as can be seen in FIG. 1, there is nothing in the conventional shielding solution to contain the radiation escaping from the bottom of the chip 12 and through the PCB 10 except for the phenomena of partial reflection from the ground plane 20 (sometimes referred to herein as an “image” plane) which can, in some situation, improve the radiation emissions problem but is problematical from a design and manufacturing point to achieve.
Therefore, what are needed are improved methods and EMI shields for preventing radiation from escaping from the bottom of the chip and through the PCB.